The long summer season is about to begin for Europe. Here, I present an outlook that begins with a sub-monthly breakdown of June, then takes an overall look at each of July and August.
Aside from some dynamical analysis at the start, the focus will be on the temperature and rainfall probabilities given by the May 2020 issue of the MetSwift Analogues. Please note that these provide guidance as to the most likely outcome, not an exact prediction of what will happen in summer 2020.
Late May into early June, a strong tropical wave will cross the tropical Pacific. Weather patterns during the first half of June 2020 will be significantly influenced by the lagged effects of this.
One way to monitor the overall atmospheric response is to look at Global Atmospheric Angular Momentum (GLAAM, or AAM for short).
The downward trend visible in this prediction from the CFSv2 dynamical model is the response to that tropical wave. Notice how it ceases mid-June – that’s when the lagged effects cease.
There’s then a strong signal for AAM to rise again, perhaps by a lot. This is likely in response to a new tropical wave being predicted but this time moving eastward from the Indian Ocean, across Indonesia. The location makes all the difference! If it takes place, it should force some big pattern changes by the end of the third week to the month.
The Influence of Angular Momentum
So, what do these AAM trends suggest for the weather patterns affecting Europe in June 2020? The following maps show composites made up of historical cases where AAM fell (left) then rose (right) while never straying far from neutral (i.e. 0 anomaly in the plot above).
The near-term fall in AAM will likely facilitate a wetter period of weather for most of northern Europe, compared to what’s been experienced in late May. This should mean an end to the long run of dry weather that has been affecting many parts of the UK for a month or more. I’m sure there are many farmers and gardeners who will be pleased about that, despite the lower temperatures!
Meanwhile, dry weather should be plentiful across southern Europe and temperatures may rise above the seasonal average, particularly in Portugal and Spain.
Once AAM rises again (…assuming it does!), the weather pattern is likely to change considerably, with above-normal temperatures across most of Europe (big heatwave potential). Spain, Portugal, and western Turkey are notable exceptions. Dry weather predominates for the ‘middle belt’ of Europe (UK to central-Eastern Europe), while other areas see near or above-normal rainfall.
Signs are, this could well be the peak of dry and hot conditions this summer for that ‘middle belt’ and of wet weather for the far-south of Europe (more on that later).
Comparison with the MetSwift Analogues
The analogues suggest a warm June overall (at least 1°C above the long-term average), rather than a cool one (at least 1°C below), is most likely for much of northern Europe. They also favour near normal rainfall (within 25% of the long-term average) for most of this region, but below average (75% or less) in the UK.
Overall, this suggests that the response to rising AAM has historically tended to outweigh the one to falling AAM. Given the current model projections for AAM, it may be that 2020 is more balanced between the two states, resulting in a near-average month for many areas. Note that this is a case of contrasting conditions cancelling out, not persistently ‘normal’ weather. Heatwaves may feature widely in late June.
The probabilities for temperature are notably high for a warm month in most of Norway and northern Sweden (largely down to climate change along with a positive Atlantic Multidecadal Oscillation; AMO). A cool month is slightly favoured for the area encompassing Poland, Latvia and Ukraine and countries in between. There is a developing La Niña event in the tropical Pacific, for which historical cases show a similar tendency for cool temperatures in central-eastern Europe.
For rainfall, the analogues suggest a high risk of a wet month (125% or more of the long-term average rainfall) for north-eastern Spain, most of Germany, Czech Republic and Austria, and western Slovakia. Meanwhile, they favour a dry month for much of southernmost Europe, especially southern Portugal and Spain, Greece, and western Turkey.
This pattern shows historical correspondence to each of a positive AMO and a negative Indian Ocean Dipole (IOD). Both are being predicted for July 2020.
For this month, the most probable temperatures flip from cool to warm for central-eastern Europe. This warm signal extends as far southwest as Romania. Otherwise, a near-average month is most likely. Bear in mind that that’s the 1981-2010 average, which is a bit cooler than has been typical of e.g. 2010-2019.
For rainfall, we can clearly see an overall pattern of wet favoured in northern Europe, dry in southern. The persistence of a dry signal for the far-south through Jun-Aug suggests a high risk of drought conditions developing there during summer 2020.
This temperature and rainfall combination for August is, in fact, part of the climate trend for that month of the year. There has been an increased frequency of strong westerly flow dominating the August weather in northern Europe, driving low pressure systems across there while ‘trapping’ high pressure across southern Europe. In the UK for example, a very warm and dry August has been hard to come by so far in the 21st Century, with 2003 the only one for most places!
This likely has a lot to do with the AMO; it’s been positive for most of the time since 1998 and shows a strong correlation with such weather patterns in August.
A Possible Wildcard: Slowing North Atlantic Current
In recent years, there has been some research indicating that a major transport of warm water from the tropics to the north-eastern Atlantic has begun to slow down.
Possibly related to this is a large swathe of much cooler than usual water in the central North Atlantic that has shown great persistence since 2015.
This 2015 research paper identified a link between this and CMIP5 climate projections that predict, for the summer season, more in the way of high pressure in a region centred just west of the UK.
This could be a wildcard that encourages a drier outcome in summer 2020 for the UK and northern France. I intend to explore this further in my next blog entry – it has big implications for UK water management.
Head Meteorologist at MetSwift
Cover Photo by Unknown Author is licensed under CC BY-SA